The present invention relates to a method and apparatus for tuning a voltage controlled oscillator (VCO) within a two-point modulated phase lock loop (PLL) system.
Phase lock loop (PLL) modulated transmission systems are known in which a carrier frequency, set by a VCO as the center frequency of the loop, can be modulated by applying a signal to a single point in the loop.
Single point modulation circuits modulate the VCO by connecting the modulating signal to one side of the loop filter or the other. With the pre-loop filter connection circuit, the modulating signal is low-pass filtered by the loop as it is modulated onto the carrier. Thus components of the modulating signal that are higher than the loop bandwidth will be suppressed. Unless there is no significant energy in the modulating signal above the loop bandwidth, the signal will suffer some loss (distortion) during the modulation.
With the post-loop filter correction circuit, the modulating signal is high-pass filtered by the loop as it is modulated onto the carrier. Thus components of the modulating signal that are lower than the loop bandwidth will be suppressed. The PLL output signal will suffer some loss (distortion) during the modulation unless there is no significant energy in the modulating signal below the loop bandwidth.
Two point modulation schemes address signal loss or distortion due to loop filter effects. In a two point modulation system, the modulating signal may be applied to the loop at two placesxe2x80x94both before and after the loop filter. Assuming the gains in these two modulation paths are matched, the low-pass response seen by the pre-loop filter connection and the high-pass response seen by the post loop filter connection cancel each other out with the result being that the loop has a flat frequency response (same gain over all frequencies) for the modulating signal, avoiding distortion in the modulation signal as it is modulated onto the carrier. However, when the gains are not matched, the low-pass and high-pass responses do not cancel each other out and some distortion occurs. To avoid loss or distortion of the modulating signal as it is modulated by the loop onto the carrier, the gains of the two modulation paths must be matched. Gain matching has been accomplished in a variety of ways.
One technique to match gains first requires testing of each device incorporating a PLL then the special manufacturing or physical modification of each PLL""s loop filter resulting in a unique loop filter for every IC and PLL combination. The special manufacturing of loop filters is time consuming and very expensive.
In another technique, tuning has been accomplished by writing a value into non-volatile memory that sets the gain in the Vkmod path so that it matches the gain in the loop correction signal Vkvco path. The non-volatile memory technique was not satisfactory as it consumed large amounts of chip area on device ICs and was not capable of tuning multiple carrier frequencies or channels after the initial manufacturing of the device, and thus added great cost to device manufacturing. The non-volatile memory technique also added additional testing time in the factory for each and every device manufactured, not just a particular model, adding to manufacturing costs.
The loop filter modification technique and the non-volitile memory technique also were highly unsatisfactory in that the tuning was accomplished once at the factory for a single channel. This single channel tuning matches Vkvco and Vkmod gains for only a single channel since the gains vary for each channel. Thus, gain mismatch worsens when any channel besides the tuned channel is used resulting in varying degrees of distortion that limited the performance and capabilities of a given device.
Another tuning method added a second internal VCO and PLL which is used as a reference during power-up of the associated transmitter for each burst of transmitted data. The difference between Vkvco and Vkmod path gains is detected to determine a value that sets gain in the Vkmod path so that it matches gain in the Vkvco path. The duplication of the PLL also consumed large areas of device ICs, was not useful in portable devices due to undesirable power consumption requirements and was much more complex and costly to manufacture.
Accordingly, it would be advantageous to have a method and apparatus for tuning the gain of the Vkvco and Vkmod signal paths with lower power consumption, fewer components and a simplified and time efficient tuning arrangement.
The invention presents a method and apparatus for tuning a two-point modulated phase-locked loop (PLL) such as may be used for modulated signal transmission in a communication system. The present invention provides tuning of a PLL for single or multiple carrier frequency channels with gain correction of an applied modulation signal. The Vkvco loop correction signal is sampled and held after applying a +1 modulation signal to the modulation input and then sampled with a xe2x88x921 modulation signal applied to the modulation input. The sampled Vkvco values are compared to produce a gain correction signal which is scaled onto a modulation signal thereby producing a gain corrected modulation input signal Vkmod to the VCO. The gain through the two modulation paths to the VCO is matched using the correction signal to minimize distortion of the VCO output.
For a multiple channel transmission system, this method can be applied independently for each channel. Vkvco signals can be sampled for each channel to produce an associated gain correction signal for each channel.
These and other features and advantages of the invention will be better understood by reference to the detailed description below which is provided in conjunction with the accompanying drawings.